A New Zealand aviation specialist backs theories pilot suicide is most likely reason for the disappearance of Malaysia Airlines flight MH370 with 239 people on board.

Satellite data confirmed the plane had crashed in a remote part of the southern Indian Ocean, killing all on board, among them New Zealanders Paul Weeks, 38, and Aucklander Ximin Wang, 50.

The wife of pilot Zaharie Ahmed Shah is to be interrogated, as suspicions grow that he may have been in a poor mental state, and hijacked the flight.

Speaking to Fairfax Media before those revelations, Auckland rescue helicopter pilot Darryl Sherwin suggested all the indications were that it was a pilot suicide.

Sherwin has been offering expertise on missing aircraft since 2005, and that year pinpointed the wreckage of liquor baron Michael Erceg's helicopter near Raglan.

Why had MH370 travelled so far? It's simple, Sherwin said.

"The whole reason he's gone to this trouble is to hide the aircraft," he said.

"Pilot suicide by disgruntled employees has been proven in the past by investigators finding the black box and voice recorders.

"In all other cases of pilot suicide, if one pilot leaves the cockpit it's crashed immediately. In this case it's gone on for seven hours and you have to ask yourself why."

His conclusion was the pilot intended to crash the plane where it couldn't be picked up by radar.

When he crunched the numbers he found MH370 had been travelling slower than a normal airliner in a cruise mode to reach where its flight ended.

"When I did a calculation of the time it took to get there, I found the only reason it had gone that slow, and low, would be to hide.

"Why did he turn left? If you went right, it would have taken him through the Philippines and Sumatra and he would have gone into radar coverage.

"The whole thing has been meticulously planned."

The pilot's marriage had deteriorated and he was no longer in a relationship with his wife, Faizah Khan, British newspaper the Mirror said.

His unstable domestic situation together with his support of a political opposition leader recently jailed in Malaysia meant his background was coming under close scrutiny.

Police were also examining reports he received a two-minute phone call shortly before takeoff from a mystery woman using a mobile number obtained using a false identity.

Faizah Khan faced questioning in an investigation supported by the FBI. A source close to the probe said vetting on co-pilot Fariq Abdul Hamid was almost finished and had so far found nothing untoward.

According to the International Business Times, authorities said they had delayed questioning Faizah Khan because it was not appropriate for people in situations of terrible bereavement to face the stress of intensive questioning.

The New Zealand Herald quoted an unnamed fellow pilot as saying the pilot of MH370 could have taken the Boeing 777 for a "last joy ride". He had been "terribly upset" when his wife told him she was leaving and he might have decided to take the plane to a part of the world where he had never flown.

Zaharie Shah had also been having relationship problems with another woman he was seeing.

Malaysia has said it has narrowed the search for the plane to an area the size of Texas and Oklahoma in the southern Indian Ocean, while the Mail Online said suicide had become the most likely cause of the flight's disappearance.

A senior industry source believed the plane was deliberately flown to an extreme altitude to knock out the passengers.

Shortly after the last voice communication from the cockpit of the plane on March 8 it was tracked by military radar flying between 43,000 and 45,000 feet (13,106 and 13,716m).

The source said the plane was tracked flying at that altitude for 23 minutes. Oxygen would have run out in a de-pressurised cabin, rendering passengers unconscious.

The team investigating the Boeing 777's disappearance believe no malfunction or fire was capable of causing the aircraft's unusual flight or the disabling of its communications system before it veered wildly off course on a seven-hour silent flight into the sea. An analysis of the flight's routing, signalling and communications showed it was flown "in a rational way".

An official source told the Telegraph that investigators believed "this has been a deliberate act by someone on board who had to have had the detailed knowledge to do what was done ... Nothing is emerging that points to motive."